Add on_flood() callback.

This callback is called if a liquid definitely floods a non-air
node on the map. The callback arguments are (pos, oldnode, newnode)
and can return a `bool` value indicating whether flooding the
node should be cancelled (`return true` will prevent the node
from flooding).

Documentation is added, the callback function was tested with a
modified minetest_game.

Note that `return true` will likely cause the node's `on_flood()`
callback to be called every second until the node gets removed,
so care must be taken to prevent many callbacks from using this
return value. The current default liquid update interval is 1.0
seconds, which isn't unmanageable.

The larger aim of this patch is to remove the lava cooling ABM,
which is a significant cost to idle servers that have lava on their
map. This callback will be much more efficient.

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